Automatic temperature control for electric heaters



June 1958 w. H. ELLIOT ETAL I 2,838,543

AUTOMATIC TEMPERATURE CONTROL FOR ELECTRIC HEATERS Filed July 2. 1954 2 Sheets-Sheet l (5. l 'w rw.

June 10, 1958 w. H. ELLIOT ETAL 2,838,643

AUTOMATIC TEMPERATURE CONTROL FOR ELECTRIC HEATERS Filed Ju1y'2. 1954 2 Sheets-Sheet 2 AUTOMATHC TEMPERATURE CONTROL FOR ELECTRIC HEATERS William 1-1. Elliot, Whitefish Bay, and Edwin X. Schmidt,

Nashota, Wis, assignors to Cutler-Hammer, Inc Milwaukee, Wis, a corporation of Delaware Application July 2, 1954, Serial No. 441,133

8 Claims. (Cl. 219-20) This invention relates to automatic temperature control for electric heaters.

It is the principal object of the present invention to provide an improved form of temperature control for electric heaters which is characterized by its relative simplicity.

in carrying out the invention a resistance bridge network is employed wherein the heater to be controlled or a resistance thermometer forms one leg thereof, and wherein an electron discharge tube responsive to the potential difference across a diagonal of the) bridge regulates the heat input to the heater or thermometer to maintain it substantially at a predetermined temperature.

()ther objects and advantages of the invention will hereinafter appear.

in the drawings:

Figure 1 is a diagrammatic showing of a control system for an electric heater incorporating the invention,

Fig. 2 depicts a slightly modified form of control system,

Fig. 3 depicts another modified form of control system, and

Fig. 4 depicts still another modified form of control system.

Referring to Fig. 1, it shows a source of single phase A. C. supply comprising lines L1 and L2. An electrical resistance heater ill is connected in series with a resistor 12 across lines L1 and L2. A resistor 13 is connected in series with an adjustable resistor 14 across lines L1 and L2. A gaseous electron discharge tube 15, having a cathode 15 anode 15 control electrode 15 and screen grid 15 has its anode 15* connected in series with an adjustable resistor 16 to line L1, and has its cathode connected to the junction 17 common between resistors and 12. The control electrode is connected in series with a resistor 18 to the junction 19 between resistors 13 and 14, and is also connected in series with a filter capacitor 20 to cathode 15. Screen grid 15 is connected to cathode 15.

Heater 153 is formed of a metal having a positive temperature coenicient of resistance, and the resistors 12, 13 and are preferably formed of materials having substantially a zero temperature coefficient of resistance. it will be seen that heater 1t) and resistor 12, 13 and 14 form a bridge circuit with each of such elements comprising a leg thereof. The potentials of the junctions 17 and 19 bear a relationship to each other which is a function of the relative resistance values of the four legs of the bridge.

Let it be assumed that at the time power is initially applied to lines L1 and 12 the potential of junction 17 is negative with respect to junction 19. As tube 15 will conduct when its anode and control grid are positive with respect to its cathode, it will be initially conducting during alternate half-cycles, and during such conducting half-cycles heater 10 will be supplied with additional current through tube 15 with the amplitude of such addi- States Patent 0 tional current being dependent upon the setting of the adjustable resistor 16. The additional current supplied through the tube causes the temperature of heater 10 to rise and its resistance increases accordingly. Consequently, the potential of junction 17 relative to that of junct on 19 increases and ultimately will be substantially equal thereto. When the potential of junction 17 is substantially equal to, or higher, than that of junction 19, tube 15 will not conduct during the half-cycles when it is capable of conduction and thus the supplementary current supply to heater 10 will be cut-oil. Due to heat transfer to the surrounding medium, heater 10 will then decrease in temperature with attendant decrease in resistance thereof. When the resistance of heater 10 decreases sufficiently, the potential of junction 17 will be below that of junction 1h and tube 15 will then conduct again during its conducting half-cycles. It will be apparent that tube 15 will repeatedly cycle on and oii; its period of on-to-ofi depending upon the rate of heat transfer from heater 10 to the surrounding medium. Adjustable resistor 16 permits adjustment of the effective amount of supplementary current supplied to heater 10 during each half-cycle when tube 15 is actually conducting, and can be utilized to adjust the rate of temperature rise of heater it) to suit particular ambient conditions.

In one embodiment constructed in accordance with Fig. 1, the various components were selected to be of the following types or ratings:

Heater iii nickel wire thermometer,

54 ohms at 72 F.

Resistor 12 600 ohms Resistor 13 1810 ohms Resistor 14 500 ohms (adjustable) Resistor 16 500 ohms (adjustable) Resistor 18 160,600 ohms Tube 15 Electrons C1] /A Thyratron With resistor 14 adjusted to aiiord 199 ohms eficctive resistance, it was found that with this particular embodiment, heater 1%) could be maintained very close to F.

In the embodiment of Fig. l, the temperature of heater it) which the system functions to maintain can be altered by changing the ohmic values of any one of the resistors 12, 13 and 14. If resistor 14 is adjusted to provide greater effective resistance, or if the value of resistor 13 is decreased, the control point temperature of heater 10 which the system functions to maintain will be increased. Conversely, if resistor 14 is adjusted to provide a lesser effective resistance, or the value of resistor 13 is increased, such control point temperature will be decreased. Increasing the value of resistor 12 will increase such control point temperature, and decreasing the value of resistor 12 will result in decrease thereof.

While not limited thereto the embodiment of Fig. 1 has been found useful for controlling the temperature of resistor 19 when used as a catalyst. It is also useful for controlling the heat of resistor 10 when used as a heating element in an electric blanket or pad.

Fig. 2 depicts a slightly modified form of the system of Fig. 1, wherein the resistor 12 and 13 instead of being connected directly to line L1 of the A. C. source are connected through a half-wave rectifier 21 to such line; rectifier 21 being connected in the same conducting relation in the circuit as tube 15. With such modified form of the control system heater 10 will be supplied with current only during the half-cycles when tube 15 is capable of conduction. Thus, in the embodiment of Fig. 2, tube 15 will supply a greater amount of the total wattage input to heater 1b which may be an advantage in certain applications where increased sensitivity of the control system is desirable.

Fig. 3 depicts an embodiment wherein a measuring a saturable reactor 33 to line L2.

, 3 resistance thermometer 30, disposed in a calorimeter air conduit 39 is substituted in the bridge circuit between junction 17 and line L2, and an electric resistance heater 32, also disposed in conduit 39 upstream from thermometer 30, is connected at one end to line L1 and at its other end in series with'A. C. windings 33 and 33 of Tube has its cathode 15 connected in series with the D. C. control winding 33 of reactor 33 to junction 17, and a half-wave rectifier 34 is connected across winding 33 in the conducting relation shown. Thermometer has a positive temperature coeflicient of resistance, and reactor 33 is preferably so selected that the impedance of its windings 33 and 33* will be very high with zero voltage across its control widing 33. Flow of pulsating direct current through winding 33 reduces the impedance of winding 33 and 33', thereby increasing the current flow through heater 32. Heat is transferred from heater 32 to resistor 30 thereby increasing the temperature and resistance of the latter to afford balance of the bridge network and establish a substantially constant temperature for resistor 30. The arrangement is advantageous where relatively high amounts of heat input to the air or other medium is required.

Fig. 4 depicts a modified system wherein an electric resistance heater 40, located in an air conduit 41 and having a zero, temperature cocificient of resistance, is connected between junction 17 and line L2 and an electric resistance thermometer 42, located downstream in conduit 41 from heater and having a positive temperature coefficient of resistance, is connected between line L1 and junction 19. It will be apparent, that resistor 12 could be used in place of resistor 42 as the sensing thermometer, in which case it should be made of a negative temperature coeflicient material.

We claim:

1. In combination, a bridge circuit comprising two parallel pairs of series connected resistors for connection across a source of alternating potential, and a gaseous electron discharge tube having an anode for connection to one side of said source, a cathode connected in circuit with the junction intermediate one pair or" series connected resistors and a control electrode connected to. the junction intermediate the other pair of resistors, said resistors being selected with respect to their normal resistance values and their temperature coefiicients of resistance so that when said tube and said bridge circuit are initially connected to such a source the ratio of the resistance or the resistor of said one pair of series connected resistors which would be connected to the other side of such source to the total resistance of said one pair of resistors is greater than the ratio of the resistance of the resistor of said other pair of resistors which would be connected to the last mentioned side of such source to the rotal resistance of said other pair of resistors and so that the last mentioned ratio increases as a result of conduction of said tube.

2. The combination according to claim 1 wherein one of said resistors is adjustable.

3. The combination according to claim 1 together with an adjustable resistor in circuit with the anode of said tube for adjusting the amount of current flow through the latter when conducting.

4. The combination according to claim 1 together with a half-wave rectifier connected in series with corresponding resistors of the respective pairs of the latter for limiting current flow through said bridge circuit to half-cycles when said tube is capable of conduction.

5. The combination according to claim 1 together with another resistance element and a saturable reactor having its A. C. windings in series circuit with the last mentioned resistance element across said source and having its D. C. control winding connected in series between the cathode of said tube and the first-mentioned junction of said bridge circuit,

6. The combination according to claim 1 together with a conduit through which air flows to an air utilizingdevice and wherein at least one of the resistance elements of said bridge circuit is located for subjection to such air fiow.

7. The combination with an electric resistance heating element and a source of A. C. supply, of a circuit for supplying and controlling current flow through said heating element comprising a' resistance element connected in series with said heating element across said source,'second and third resistance elements connected in series across said source, and a gaseous electron discharge tube having an anode connected to one side of said source, a cathode connected to the junction intermediate said heating element and the first mentioned resistance element and a control electrode connected to the junction intermediate said second and third resistance elements, said heating element and said resistance elements being selected with respect to their nominal resistance values and their temperature coefiicients of resistance so that when said heating element, the resistance elements and said tube are initially connected to such source the ratio of the resistance of said heating element to the total resistance of said heating element plus that of said first mentioned resistance element is less than the ratio of said third resistance element to the total resistance of said second and third resistance elements and so that the first mentioned ratio increases as a result of current flow through said tube and said heating element.

8. The combination according to claim 1 wherein said heating element has a positive temperature coeflicient of resistance.

References Cited in the file of this patent UNITED STATES PATENTS 1,994,076 Kuhle et al. Mar. 12, 1935 2,272,492 Weyher Feb. 10, 1942 2,429,453 Crowley Oct. 21, 1947 2,440,682 Hansel May 4, 1948 2,490,965 Huck Dec. 13, 1949 2,511,981 Hanchett June 20, 1950 2,585,005 Godshalk- Feb. 12, 1952 2,646,544 Sands July 21, 1953 

